Latent thiol monomers

ABSTRACT

This invention relates to latent thiol monomers and their use in the synthesis of polymers. In addition, this invention relates to novel polymers and graft copolymers formed with these latent thiol monomers.

FIELD OF THE INVENTION

This invention relates to latent thiol monomers and their use in thesynthesis of polymers. In addition, this invention relates to novelpolymers and graft copolymers formed with these latent thiol monomers.

BACKGROUND OF THE INVENTION

"Latent thiol monomer," as used herein, is defined as a polymerizablemonomer containing; at least one functional group polymerizable to forma homopolymer or copolymerizable with at least one first ethylenicallyunsaturated monomer to form a copolymer; and at least one protectedthiol group, for example a thioester, preferably thioacetate. The term"latent" refers to the fact that the protected thiol group does notreact in the homopolymerization reaction or the copolymerizationreaction with the first ethylenically unsaturated monomer or monomers.After the homopolymerization or copolymerization is conducted, theprotected thiol group can be deprotected, for example by a cleavingreaction, to form a thiol group pendant to the polymer chain. This thiolgroup so formed can then react in a second polymerization with at leastone second ethylenically unsaturated monomer, to form a graft copolymer.

"Graft copolymers" as used herein, are defined as the macromoleculeformed when polymer or copolymer chains are chemically attached to apolymeric backbone as side chains. Generally, in graft copolymers, theside chains are of different composition than the backbone chain. Due tothe ability to chemically combine unlike polymeric segments in onemolecule, graft copolymers have unique properties, making themparticularly useful for their surface active properties, such as forexample in stabilizing physical blends of otherwise incompatiblepolymeric or monomeric compounds.

The copolymer products of the present invention can be used directly inwater-based emulsion coatings, elastomers, adhesives, caulks andmastics. Still further uses for these copolymers are as plasticadditives for use as compatibilizers of polymer--polymer blends.

DESCRIPTION OF THE PRIOR ART

Canadian Patent 821,000 discloses a process for the preparation of blockand graft copolymers. Unlike the aqueous process of the presentinvention, this is a nonaqueous process. The copolymers of CanadianPatent 821,000 are produced by first forming a backbone polymercontaining epoxide functionality, reacting this backbone polymer with amercaptan containing compound such as mercaptopropionic acid, thenpolymerizing another ethylenically unsaturated monomer in the presenceof this backbone polymer such that the mercapto groups on the firstpolymer react with the vinyl groups of the polymerizing monomer, formingblock and graft copolymers.

In addition, the process of Canadian Patent 821,000 is unlike thepresent invention in that in the present invention the latent thiolmonomer reacts, in one step, with the at least one first ethylenicallyunsaturated monomer. The polymer chain formed during the first stage ofthe emulsion polymerization does not have to be reacted with anadditional mercaptan containing compound, but can simply be deprotectedto form a polymer chain containing a pendant thiol group.

U.S. Pat. No. 2,947,731 discloses a nonaqueous process of making vinylbenzene thioesters and their subsequent homopolymerization, orcopolymerization with other vinyl monomers. In addition, it is disclosedthat the thioester groups on these polymers and copolymers can behydrolyzed to form the corresponding vinyl benzene thiol. However,nowhere is it taught or suggested to form graft copolymers, particularlyby an emulsion polymerization process.

A key drawback to several of these techniques is the use of organicsolvent as a required component of the polymerization, making itnecessary to remove the solvent if a solvent-free product is desired.Other problems with all these techniques, whether polymerized in solventor bulk polymerized using monomer as the solvent, are that they allyield a product with low molecular weight and all these prior arttechniques lead to poor conversion of monomer to polymer.

In an article by Nakahama et al., Makromol. Chem., Rapid Commun. 10,397-401(1989), and Makromol Chem. 192, 1891-1902(1991), an emulsionpolymerization of styrene with an isothiuronium salt containing monomeris reported. The salt is subsequently hydrolyzed to form polymerscontaining thiol groups. However, nowhere is the subsequent reaction ofthe thiol groups to form a graft copolymer taught or suggested. Inaddition, this cationic process results in severe compatibility problemswith most anionic species.

SUMMARY OF THE INVENTION

Latent thiol monomers are polymerizable monomers containing; at leastone functional group polymerizable to form a homopolymer orcopolymerizable with at least one first ethylenically unsaturatedmonomer to form a copolymer; and at least one protected thiol group.After polymerization or copolymerization of the latent thiol monomers,the protected thiol group can be deprotected to produce a polymer havingpendant thiol functional groups. In a subsequent or second stagepolymerization, the pendant thiol function groups react with at leastone second ethylenically unsaturated monomer to produce the graftcopolymer.

DETAILED DESCRIPTION OF THE INVENTION

Latent thiol monomers are polymerizable monomers containing; at leastone functional group polymerizable to form a homopolymer orcopolymerizable with at least one first ethylenically unsaturatedmonomer to form a copolymer; and at least one protected thiol group.When the functional group is, for example, a vinyl group, the vinylgroup either homopolymerizes or copolymerizes with at least one firstethylenically unsaturated monomer forming a copolymer. The protectedthiol group on the latent thiol monomer does not react, or if it doesreact it only reacts to a limited extent, during the homopolymerizationof the latent thiol monomer or the copolymerization with the at leastone first ethylenically unsaturated monomer. After the polymerization orcopolymerization, a polymer chain is formed with pendant protected thiolgroups.

Examples of latent thiol monomers include compounds with the followingstructure; ##STR1## where R is a monovalent organic radical havingpolymerizable vinyl or olefinic groups;

R₁ is a polyvalent organic radical;

R₂ is an acyl radical (including acetoacetyl); and

x is 0 or 1.

Additional examples of latent thiol monomers include vinyl benzylthiolesters.

Specific examples of some latent thiol monomers include; allyl3-mercaptopropionate thioacetate,(S-acetyl-3-mercaptopropyl)-2-methyl-2-propenoate,(S-benzoyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate,(S-2,2-dimethylpropanoyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-acetoxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-acetoacetoxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-tetrahydropyranoxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-(2-methoxy-2-propoxy)propyl)-2-methyl-2-propenoate,2,3-epithiopropyl 2-methyl-2-propenoate,(S-acetyl-2-mercapto-3-acetoxypropyl)-2-methyl-2-propenoate,S-acetyl-(1-allyloxy-3-mercapto-2-hydroxypropane),S-benzoyl-(1-allyloxy-3-mercapto-2-hydroxypropane) andS-2,2-dimethylpropanoyl-(1-allyloxy-3-mercapto-2-hydroxypropane). Themore preferred latent thiol monomers are(S-acetyl-3-mercapto-2-acetoxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate, and theeven more preferred is(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate.

When the latent thiol monomer is, for example, allyl3-mercaptopropionate thioacetate, it may be prepared by first reacting3-mercaptopropionic acid with allyl alcohol to form allyl3-mercaptopropionate. This can then be reacted with acetic anhydride toform allyl 3-mercaptopropionate thioacetate.

When the latent thiol monomer is, for example,(S-acetyl-3-mercaptopropyl)-2-methyl-2-propenoate, it may be prepared byfirst reacting thiolacetic acid and allyl alcohol in the presence oft-butylhydroperoxide catalyst to form a thioacetate functional alcohol.This thioacetate functional alcohol product can then react withmethacrylic anhydride to form the monomer.

When the latent thiol monomer is, for example,(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate, it may beprepared by reacting glycidyl methacrylate and thiolacetic acid. Thisreaction can be carried out in most solvents, but it is preferable tocarry out the reaction in a 50% by weight ethanol/water solvent system.Purification of the monomer by removal of residual base catalyst leadsto a more stable monomer. This can be accomplished by such techniquesas, for example, vacuum treatment, flash chromatography on silica, andfiltration through ion exchange resin. The more preferable technique isfiltration through an ion exchange resin, preferably Amberlite® IRC-50ion exchange resin (a registered trademark of the Rohm and HaasCompany).

The ethylenically unsaturated monomer useful in the copolymerizationwith the latent thiol monomer can be any ethylenically unsaturatedmonomer, for example; acrylate esters and acids; methacrylate esters andacids; acrylonitrile; methacrylonitrile; acrolein; methacrolein; vinylaromatic compounds such as styrene, substituted styrene, vinyl pyridineand vinyl naphthalene; vinyl esters of organic acids, such as vinylacetate; N-vinyl compounds such as N-vinyl pyrrolidone; unsaturatedhalogenated compounds such as vinyl chloride and vinylidene chloride;acrylamide, methacrylamide and substituted acrylamides andmethacrylamides; polymerizable sulfonic acids and salts thereof such asstyrene sulfonic acid, sodium vinyl sulfonate, sulfoethyl acrylate,sulfoethyl methacrylate and acryloamidopropanesulfonic acid (AMPS);vinyl ethers; or combinations thereof.

The latent thiol monomers of the present invention can behomopolymerized or copolymerized in all types of polymerizationreactions well known to those skilled in the art, for example in asolution or emulsion polymerization. It is preferable, when forminggraft copolymers of the present invention to use an aqueous, two stageemulsion polymerization process.

During the formation of the graft copolymer, the backbone portion of thecopolymer is formed during a first stage of the aqueous emulsionpolymerization. The backbone is formed by either the homopolymerizationof at least one latent thiol monomer or the copolymerization of the atleast one latent thiol monomer and the at least one first ethylenicallyunsaturated monomer.

The latent thiol monomer is contained in the first stage of the aqueousemulsion polymerization at a concentration of up to 100%, morepreferably up to about 20%, even more preferably up to about 10%, andeven more preferably up to about 3%, based on the total weight of themonomers in stage one.

The first stage emulsion polymerization should be run such that theprotected thiol group from the latent thiol monomer remainssubstantially intact during the first stage polymerization. In addition,it is preferable to run the first stage emulsion polymerization reactionin an inert atmosphere, for example, in a nitrogen atmosphere.

Once the polymer chain with pendant protected thiol groups has beenformed in the first stage of the aqueous emulsion polymerization, thepolymer is subjected to a deprotection reaction, for example a cleavingreaction or thermal heating, whereby the protected thiol groups (latentthiol groups) are deprotected, converting them into free thiol groups.

When the protected thiol group of the polymer chains produced in thefirst stage emulsion polymerization are deprotected using a cleavingreaction, for example when the protected thiol group is thioacetate, anycleaving technique well known to those skilled in the art may be used.However, it is preferable to cleave the thioacetate group with, forexample, ammonia, hydroxylamine, N-propylamine, diethylamine,morpholine, dimethylaminoethanol, and hydrazine. The more preferredcleaving agents are ammonia, dimethylaminoethanol and hydrazine and theeven more preferred is hydrazine. Generally, the cleaving reaction isrun at a temperature of from about 15° to 95° C. and more preferablyfrom about 65° to 75° C.

Once the protected thiol groups have been deprotected to form pendantthiol groups, the polymer chain produced in the first stage emulsionpolymerization can be isolated, for example by spray drying, used as is,or stored for further reaction at a later time. However, it is highlypreferred that the second stage monomer emulsion be added directly tothe polymer emulsion of stage one to form the graft copolymer. One ofthe key advantages of this process is that the polymer of stage one doesnot have to be isolated before reacting in stage two, and stage two cantake place simply by adding stage two monomer.

In stage two of the aqueous emulsion polymerization at least one secondethylenically unsaturated monomer, preferably in the form of an aqueousemulsion, is added to a reaction mixture containing the polymer chainformed during the first stage of the aqueous emulsion polymerization.Because the polymer chain from the first stage is essentially a transferagent containing pendant thiol groups, it is preferable to add all ofthe second stage monomer together at one time. However, if the secondstage monomer is gradually added, some non-graft copolymer may form,yielding a mixture of graft copolymer and polymer derived from secondstage monomer. This mixture may have some beneficial uses.

The at least one second ethylenically unsaturated monomer can be any ofthe ethylenically unsaturated monomers listed above for use as the atleast one first ethylenically unsaturated first monomer.

The aqueous emulsion copolymerization technique of the present inventionis based on a two stage polymerization where the mode of monomeraddition in the first stage is not critical and a single addition ofmonomer in the second stage is preferred. The aqueous emulsioncopolymerization techniques used in the present invention are well knownto those skilled in the art. The temperature of the reaction in each ofthe two stages should be in the range of from about room temperature toabout 150° C., more preferably from about 50° C. to 90° C.

An emulsifier can be used in the process of the present invention andcan be of the general type of an anionic, cationic, or nonionicemulsifier. The more preferred emulsifiers are the anionic and thenonionic emulsifiers and the even more preferred are the anionicemulsifiers, such as sulfates and sulfonates, like sodium lauryl sulfateand sodium dodecyl benzene sulfonate. The amount of emulsifier used maybe from about 0.05to 10%, and more preferably from about 0.3 to 3%,based on the total weight of the monomers. Many other emulsifiers can beused and are well known in the emulsion polymerization art.

The latex particle size is controllable to be as small as from about 50to 200 nanometers (nm) to as large as 800 nm or more by adjusting thetype and level of emulsifier used. The particle size is preferably lessthan 500 nm.

It is advantageous to initiate and catalyze the reaction in each of thetwo stages in a conventional manner. Any commonly known free radicalgenerating initiators can be used, such as persulfates, peroxides,hydroperoxides, peresters and azo compounds. Specific examples arebenzoyl peroxide, tert-butyl hydroperoxide, azodiisobutyronitrile andsodium, potassium and ammonium persulfates. The more preferred are thesodium, potassium and ammonium persulfates which can be used bythemselves, activated thermally, or in a redox system. When used in aredox system, reducing agents such as sodium formaldehyde sulfoxylate,isoascorbic acid and sodium bisulfite can be used along with a promoter,such as for example iron or others well known to those skilled in theart. Thermal initiation is more preferred. The amount of initiator willgenerally be in the range of from about 0.1 to 3.0% by weight, based onthe total weight of the monomers.

The reaction conditions used in the second stage are dependant on themethod of deprotection of the protected thiol group. For example, if acleaving reaction utilizing ammonia is used to deprotect the protectedthiol group, it is preferable to initiate the second stagepolymerization thermally using ammonium persulfate or with redoxinitiators of tert-butylhydroperoxide and sodium formaldehydesulfoxylate or isoascorbic acid. If hydroxylamine is used to deprotectthe protected thiol group via a cleaving reaction, it is preferable toneutralize the amine with, for example, acetic acid, prior to the secondstage polymerization. If hydrazine is used to cleave the protected thiolgroup, it is preferable to complex the hydrazine with 2,4-pentanedioneprior to the stage two emulsion polymerization.

Additional initiator or catalyst systems may be added after stage twopolymerization to reduce any residual monomer.

Generally, the aqueous emulsion formed containing the graft copolymerhas a solids level of from about 20 to about 60%, based on the totalweight of the aqueous composition. The graft copolymer products of thisaqueous emulsion polymerization can be isolated, for example by spraydrying, coagulation or other techniques well known to those skilled inthe art. However, it is preferable to use the aqueous emulsioncontaining the copolymer as is.

The invention will now be illustrated by the following non-limitingexamples.

EXAMPLES 1-4 Preparation of Latent Thiol Monomers EXAMPLE 1 Preparationof 2-Propenyl-(S-Acetyl-3-Mercaptopropionate) (Ally 3-MercaptopropionateThioacetate) Step 1-Preparation of Allyl 3-Mercaptopropionate

                  TABLE 1                                                         ______________________________________                                        Reagents for Example 1-Step 1                                                 ______________________________________                                        Allyl alcohol        200 g.  3.44 mole                                        3-Mercaptopropionic acid                                                                           250 g.  2.36 mole                                        Methoxy Hydroquinone (MEHQ)                                                                        1.0 g.                                                   Phenothiazine        0.5 g.                                                   p-Toluenesulfonic acid                                                                             1.0 g.                                                   Toluene              250 g.                                                   ______________________________________                                    

The reagents shown in Table 1 were mixed in a nitrogen flushed 1 literflask fitted with a Dean Stark condenser, thermometer, and magneticstirrer. The reaction mixture was heated to reflux until the theoreticalamount of water had been collected. Under a nitrogen atmosphere, theDean Stark condenser was removed and replaced with a Vigreaux column(24") with distillation head. Allyl alcohol and toluene were removedfrom the reaction mixture at reduced pressure (20 mm Hg). Thedistillation was halted before the temperature reached 85° C., thedistillation temperature at reduced pressure of allyl3-mercaptopropionate.

Step 2--Conversion to Allyl-3-Mercaptopropionate Thioacetate

The reaction mixture from step 1 was cooled under nitrogen and thendiluted with 200 g. of methylene chloride. Then, 289 g. of aceticanhydride, along with a catalyst of 0.5 g. of 4-dimethylaminopyridine,were added to the reaction mixture. The reaction mixture was stirred for1 hour at which time NMR analysis of a vacuum stripped aliquot indicatedcomplete conversion to the desired thioacetate. The product wasdistilled at 132°-134° C. at 20 mm Hg to yield 355 g. of product (80%).

EXAMPLE 2 Preparation of(S-Acetyl-3-Mercaptopropyl)-2-Methyl-2-Propenoate Step 1--Preparation ofS-Acetyl-3-Mercaptopropanol

                  TABLE 2                                                         ______________________________________                                        Reagents for Example 2-Step 1                                                 ______________________________________                                        Thiolacetic acid   160 g.  2.1 mole                                           Allyl alcohol      150 g.  2.58 mole                                          t-Butylhydroperoxide                                                                             1.8 g.                                                     ______________________________________                                    

To a 500 ml 3-neck flask equipped with thermometer, reflux condenser,addition funnel, and magnetic stirring was placed 130 g. of allylalcohol. The addition funnel was charged with 130 g. thiolacetic acidand in a syringe was placed a solution of 1.8 g. t-butylhydroperoxide(t-BHP) in 20 g. allyl alcohol. Initially, 15 g. of thiolacetic acid wasadded to the kettle along with 2 ml. of the t-BHP solution. A slowcofeed of the remaining thiolacetic acid was begun along with the slowaddition of the remaining t-BHP solution so as to maintain a reactiontemperature of between 45°-55° C. Addition was complete in 1 hour.

NMR analysis of an aliquot showed only the desired thioacetate alcoholalong with residual allyl alcohol. Silver nitrate titration for residualthiolacetic acid showed essentially complete conversion. The excessallyl alcohol was stripped by a rotary evaporator and the product wasused directly in the next step.

Step 2--Conversion to (S-Acetyl-3-Mercaptopropyl)-2-Methyl-2-Propenoate

                  TABLE 3                                                         ______________________________________                                        Reagents for Example 2-Step 2                                                 ______________________________________                                        Thioacetate alcohol                                                                              280 g.  2.09 mole                                          Methacrylic anhydride                                                                            400 g.  2.59 mole                                          Tetrahydrofuran (THF)                                                                            450 g.                                                     Phenothiazine      2.0 g.                                                     4-dimethylaminopyridine                                                                          2.0 g.                                                     ______________________________________                                    

The reagents listed in Table 3 were added to a 2 liter round bottomflask and the mixture was heated to reflux for 5 hours. The product wasfractionally distilled at reduced pressure (1-3 mm Hg) through anOldershaw column (30 in). In the initial distillation the fractionboiling between 80°-105° C. was collected. This fraction was thendistilled a second time with the material boiling at 87°-94° C. (2 mmHg) being collected. NMR analysis of this fraction showed minorimpurities (5%) and the desired(S-acetyl-3-mercaptopropyl)-2-methyl-2-propenoate (249 g.; 60% yield).

EXAMPLE 3 Preparation of(S-Acetyl-3-Mercapto-2-Hydroxypropyl)-2-Methyl-2-Propenoate

                  TABLE 4                                                         ______________________________________                                        Reagents for Example 3                                                        ______________________________________                                        Glycidyl methacrylate (GMA)                                                                         300 g.  2.11 mole                                       Thiolacetic acid      159 g.  2.09 mole                                       Ethanol               350 g.                                                  Water                 300 g.                                                  Butylated hydroxy toluene (BHT)                                                                     2.0 g.                                                  Ammonia (28%)         0.5 g.                                                  ______________________________________                                    

To a 2 liter 4-neck flask fitted with a mechanical stirrer,thermocouple, and reflux condenser was added in the following order: 1)glycidyl methacrylate, 2) ethanol containing BHT, 3) water, 4)thiolacetic acid and 5) ammonia. Upon addition of the ammonia, thereaction began to exotherm slowly, the temperature rising at about 0.5°C./minute for the first 10 minutes, and increasing to 1° C./minute overthe next 30-40 minutes. The reaction temperature peaked at 68°-72° C.and then began to cool.

NMR analysis of a vacuum stripped sample showed essentially completeconversion to(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate. Silvernitrate titration for unreacted thiolacetic acid indicated greater than99% conversion of the thiol.

The product was pumped through a column of Amberlite IRC-50 weaklyacidic resin (100 g. dry weight). The filtered product was stored at 5°C. where it exhibited less than 5% decomposition in 1 month.

EXAMPLE 4 Preparation ofS-Acetyl-(1-Allyloxy-3-Mercapto-2-Hydroxypropane)

                  TABLE 5                                                         ______________________________________                                        Allyl glycidyl ether                                                                            40 g    0.35 mole                                           thiolacetic acid  30 g    0.40 mole                                           triethylamine     0.25 g                                                      tetrahydrofuran   100 g                                                       ______________________________________                                    

Allyl glycidyl ether and thiolacetic acid where dissolved intetrahydrofuran and the triethylamine catalyst was added. The mixturewas heated to reflux for 40 minutes at which time NMR analysis indicatedcomplete conversion toS-acetyl-(1-allyloxy-3-mercapto-2-hydroxypropane).

EXAMPLE 5 Emulsion Polymerization Stage One

Preparation of emulsion copolymer of 96.5 parts Butyl Acrylate/2 parts(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate/1.5 partsMethacrylic Acid

To a 3 liter, 4 necked flask fitted with reflux condenser, thermometerand mechanical stirrer was added 570 g. of water and 7 g. of a 2.3%aqueous solution of sodium dodecylbenzenesulfonate. A monomer emulsionwas prepared consisting of; 200 g. water; 10 g. of a 23% aqueoussolution of sodium dodecylbenzenesulfonate; 675.5 g. of butyl acrylate;14 g. of (S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate(from Example 3); and 10.5 g. of methacrylic acid. A portion of thismonomer emulsion (91 g.) was added to the kettle and the reactionmixture was then heated to 80° C. A solution of 1.0 g. ammoniumpersulfate in 34 g. of water was then added. After the initial exothermsubsided, the monomer emulsion was added to the kettle over 2.5 hours.The kettle was maintained at 80° C. for an additional 30 minutes andthen cooled to 60° C. Then 0.4 g. of t-butylhydroperoxide in 10 g. ofwater followed by 0.3 g. of sodium formaldehyde sulfoxylate in 10 g. ofwater was added. The theoretical yield was 45.5% solids and the actualyield was 45.4% solids.

Deprotection of Stage One Copolymer via a Cleaving Reaction

The stage one latex prepared above, 96.5 parts butyl acrylate/2 parts(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate/1.5 partsmethacrylic acid (45.4% total solids) 400 g. solids, was placed in a 3liter 4-necked flask (8 g., 0.037 mole of latent thiol groups present).The apparatus was then flushed with nitrogen. Hydrazine (2.0 g., 0.0625mole, 1.69 equiv.) was added and the reaction mixture was heated to 70°C. After 1 hour, silver nitrate titration of a 0.25 g. solids aliquotshowed quantitative liberation of thiol. Then, 2,4-pentanedione (6.88 g.0.06875 mole) was added to complex with the hydrazine.

Stage Two

Emulsion Polymerization of 50 parts (96.5 parts BA/2 parts(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate/1.5 partsMAA)//50 parts Methyl Methacrylate

Once deprotection was complete, the second stage monomer emulsion wasprepared:

    ______________________________________                                               MMA            400 g.                                                         Sipon WD       0.7 g.                                                         Water          500 g.                                                  ______________________________________                                    

The emulsion was added to the latex and the temperature allowed toreturn to 60° C. Ferrous sulfate/EDTA solutions (1 ml of 0.15% solution)were added and the single shot polymerization was initiated by theaddition of t-butylhydroperoxide (1.0 g. of a 70% solution in 10 g.water) followed by isoascorbic acid (1.37 g. in 10 g. water). Anexotherm of 27° C. was observed over a 10 minute period. The reactionwas allowed to cool to 60° C. and then 0.3 g. of t-BHP solution/5 g.water and 0.3 g. sodium formaldehyde sulfoxylate/5 g. water was addedtwice.

We claim:
 1. A latent thiol monomer selected from the group consistingof allyl 3-mercaptopropionate thioacetate,(S-benzoyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate,(S-2,2-dimethylpropanoyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-acetylpropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-hydroxypropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-acetoacetylpropyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-tetrahydropyranyl)-2-methyl-2-propenoate,(S-acetyl-3-mercapto-2-(2-methoxy-2-propoxy))-2-methyl-2-propenoate,(S-acetyl-2-mercapto-3-acetylpropyl)-2-methyl-2-propenoate,S-acetyl-(1-allyloxy-3-mercapto-2-hydroxypropane),S-benzoyl-(1-allyloxy-3-mercapto-2-hydroxypropane) andS-2,2-dimethylpropanoyl-(1-allyloxy-3-mercapto-2-hydroxypropane).